by Leo Marks

Reviewed by Arnold Reinhold

Leo Marks has written an entertaining, yet historically
significant autobiography of his wartime experiences working for the
Special Operations Executive S O E in England. Is remarkable in part
because of his ability to remember his attitudes as a 22 year old
junior officer even though he is now in his 80's. (Marks became a
screenwriter of note after the war. His
filmography at
www.imdb.com lists 9 writing credits and several acting roles.) A
major subtext of this work is Mr. Marks' "coming of age" in the midst
or World War II. However I will focus on the cryptological aspects of
his book.

Cryptography was a childhood hobby for Marks. Family connections
got him into a training class for Britain's famed cryptography
establishment, but a bad case of attitude cost him a slot at
Bletchley Park and instead he was sent off to the Special Operations
Executive (SOE) in downtown London. His new employers were distressed
at how long it took him to decoded a message until they realized he
didn't know he had been supplied with the key. He in turn is
horrified at the simplicity of the codes that SOE is using to
communicate with secret agents in Europe: a double transposition
cipher keyed by five words from a memorized poem. The five words
differ for each message and are indicated in a prefix to the message.
His first step to improve the security of SOE's agent communications
is to discourage agents from using classic poetry and get them to use
poems Marks and others make up instead. This prevents German
cryptographers from guessing the poems using the five word key they
recover from cracking a single message. Marks includes several
examples of his rhymes. One possible title for this book might have
been "War Poet."

Marks quickly realized that the real danger from messages being
read by the Germans was not just the exposure of one message's
content, but the ability of the Germans to figure out the security
checks the agents employ. SOE instructed agents to make certain
predictable "mistakes" in each messages. If they were captured and
forced to continue sending messages, they should simply omit those
mistakes, thereby alerting the British to their capture. In fact Mr.
Marks soon finds out that a large amount of the traffic coming from
Europe, especially the Netherlands, does not have valid security
checks. The people in charge of running those agents assure him that
this is due to sloppiness and lapses in training rather than actual
capture. Sadly, they were wrong.

Indecipherables

In addition to worrying about code security, Mr. Marks devoted
much of his energy to the problem of "indecipherables." These are
messages from agents that do not decode properly, usually due to
coding errors or garbles in transmission. If the messages cannot be
read, the agents were instructed by radio to recode and send them
again, puting those agents at much greater risk of detection and
capture. Mr. Marks puts his code breaking skills to good use and ends
up turning his code clerks into a small cryptanalysis unit to decode
these messages using related key attacks. He earns great respect from
the agents who understand he is saving their lives.

Marks is at first stymied by indecipherable messages that come
from the Free French. The French insisted on using a code supplied by
General de Gaulle and kept secret from the British. But our smart
aleck hero was more concerned with agent safety than diplomatic
sensibilities and managed to solve this problem without anybody
losing face.

Mr. Marks next major idea toward improving cryptographic security
is to replace the poems with a list of single-use transposition keys
. This goes against the time-honored doctrine that keys should be
memorized, but the Germans were were known to use torture to extract
keys and it was assumed that few agents would resist for long. The
printed (on silk) key list offers two advantages. First the agents
can do their coding more quickly and more accurately since they don't
have to deal with the complicated and error prone conversion of poems
into transpositions. In addition the agents can destroy keys that
have already been used, preventing the Germans from reading past
messages in the event of capture. In modern cryptography, this
concept is termed "forward security,") and it also allowed the agents
to use less ambiguous security checks, since the Germans could not
compare them to earlier messages.

One time pads

Mr. Marks the third innovation was to recommend to the use of one
time pads. Marks knew that one time pads offer absolute security
against cryptanalysis. They had another advantage in that messages
can be short. To ensure the SOE transposition cipher had adequate
security required a minimum message size of 200 characters. That kept
agents on the air longer and increased their risk of detection.

But Marks is familiar with one time pads only in the context of
"mainline" communications codes where the one time pads are all
digits. It takes him a while to figure out how to use letters in one
time pads, but he eventually gets it. One thing I found puzzling is
that he used a complex of transposition table to combine the letters
in a message text with the letters in the one time pad. (Several
examples of the table are shown in the photo section.) He can be
forgiven for not realizing that simple modulo 26 addition (a+a=a,
a+b=b, b+b=c, ...) would suffice and could be memorized or recreated
in the field. What is now baby level cryptography was top-secret
than. But if it appears that the folks at Bletchley Park approved his
system. Using the transposition table increased the amounts of secret
material each agents had to carry into the field on necessarily
without adding any security.

Another cryptographic puzzle in the book is a system Marks says he
developed that allowed a master agent to give agents recruited in the
field a security code that the master agent could not compromise in
the event of his later capture. Britain still considers this
technique classified and asked him not to reveal its details.

The Netherlands catastrophy

A main cryptologic theme in this book is Marks' growing
realization that something was seriously wrong in the Netherlands
section and his apparent inability to convince others of the extent
of the disaster. Before it was over, 54 agents were captured, 47 of
whom were shot and 95 parachute drops fell into waiting German hands
carrying tons of guns, explosives and ammunition. David Kahn refers
to this episode as "the worst Allied defeat in the espionage war."

Marks puts this tragic story in a different context: the
bureaucratic battle between SOE and the main British intelligence
service that Marks knows as "C." Apparently C believed that SOE was
ineffectual and wanted to disband them or at least take them over.
Evidence that the Netherlands operation was compromised would have
been all the ammo they need. So in a classic bureaucratic maneuver,
Marks is instructed to look for evidence beyond the security check
failures. It dawns on him that the Netherlands is the only country
that does not generate indecipherables. To him, this suggested
efficient German clerks were encoding the messages, not bona fide
agents, but his superiors professed to be unconvinced. He comes up
with other ploys, including getting his radio operators to send "HH"
(a German procedure sign for "Heil Hitler") in Morse code at the end
of a message. The "agent" reflexively returns the HH, but Marks
superiors are apparently still not convinced.

Phillipe Ganier-Raymond's "A Tangled Web," Arthur Barker Ltd.,
1968, tells the same story from the perspective of the agents in
Holland and the Germans themselves, including Marks' counterpart,
Major Giskes, who survived and is interviewed. The tone of this book
is appropriately darker and Ganier-Raymond devotes his last chapter
to the bitter questions of the Dutch survivors and several possible
answers. British bureaucrat infighting is not among them.

Another Explaination

I believe there is yet another possible explanation for the
continuation of the Netherlands fiasco in the face of Marks'
evidence. The British had a big secret of their own to protect: they
had cracked many of the German's codes. And Britian was running a
very complex deception too. It had penetrated the German spy network
in England even more throughly than the Germans had compromised SOE's
in Holland. But the British use of their counter espionage asset was
far more strategic. They employed it to mislead the Germans about the
coming invasion of Europe. They did this by having the notional spies
they ran send numerous reports of troop sightings that the German
intelligence could assemble into map of all the Allied forces
stationed in Britain. They expected, correctly as it turned out, that
this map would convince the Germans that the invasion was planned for
Calais instead of Normandy.

This deception was so subtle and effective that the Germans
assumed the actual invasion in Normandy was a diversion, and kept
several divisions in reserve awaiting the main Calais invasion.
Germany awarded the British-run spies the Iron Cross for their
excellent work and continued to employ the network to refine the aim
of V-2 rockets, allowing the British to divert the V-2s away from
London to more rural areas.

While the Netherlands fiasco may have started with incompetence
and may have been covered up initially by bureaucratic infighting, it
was very much in the Allies' interest to have the German intelligence
establishment and high command believe the British were a bunch of
cryptographic nincompoops. If the Britts could be so easily fooled,
surely the German codes were safe and the German spy net in England
must be operating undetected. In the cruel calculus of war, 54 agents
and a few thousand guns and supplies are a cheap insurance policy for
what the Allied Normandy deception accomplished. It would be
interesting to know if Marks' report made its way up the chain of
command to the central deception committee. Unfortunately, most of
SOE's records were destroyed after the war in a fire, so we may never
know the true story.

Bletchley Park

Another subplot in this book is Marks' relationship with Bletchley
Park. Bletchley was part of C so Marks had to be careful what he told
them, but it is clear that they looked kindly on his efforts. At one
point Marks decides to send one time pad traffic with poem code
headers in order to tie up any German cryptographers who would try to
crack that traffic. He gets a visitor from Bletchley who is a bit
cross with him for not sharing his clever idea with them. I strongly
suspect Bletchley was monitoring his traffic and wasted some time
themselves trying to break these messages. If they were monitoring
SOE agent traffic they may have had independent reason to suspect the
Netherlands operation. Again, we may never know.

Conclusion

Leo Mark's book will entertain anyone who enjoys spy novels and is
"must read" material for anyone interested in cryptography. It is now
available in
paperback.